While loop when variable is not defined

Question

Scenario. I have this code:

regexp="[^0-9A-Za-z]+"
while [[ $var =~ $regexp ]]
do
{
    var=$(dialog --inputbox "File name (number and letters only): " 12 60 --stdout)
}
done
echo $var

Condition:

The variable var should not be set (is not set) before while statement;

Target:

Leave while statement only when the variable has alphanumeric values.

But if I try to run it don't work. It works if I set up variable var, e.g:

regexp="[^0-9A-Za-z]+"
var="_"
while [[ $var =~ $regexp ]]
do
{
    ...

Now, how to run that code without set up variable var?

There is an error in your logics here: you must initialize the var before while or place this while into while true; ... done. The regex to match 1 or more alphanumeric characters is [0-9A-Za-z]+. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Nov 17, 2015 at 22:19

anubhava · Accepted Answer · 2015-11-17 22:23:02Z

2

You can use default variable value:

regexp="[^0-9A-Za-z]+"
while [[ ${var:-#} =~ $regexp ]]; do
   var=$(dialog --inputbox "File name (number and letters only): " 12 60 --stdout)
done
echo "$var"

${var:-#} will return # of var is null or unset otherwise it returns variable's value.

answered Nov 17, 2015 at 22:23

anubhava

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Comments

chepner · Accepted Answer · 2015-11-17 23:04:53Z

2

Instead of checking if var matches an illegal value, check that is doesn't match a legal value.

# Drop the ^ from the beginning of the character class
regexp="[0-9A-Za-z]+"
# Negate the condition with !
while [[ ! $var =~ ^$regexp$ ]]
do
    var=$(dialog --inputbox "File name (number and letters only): " 12 60 --stdout)
done
echo $var

There is also the little-seen until command (as well as a predefined character class that includes letters and numbers while respecting the current locale):

until [[ $var =~ ^[[:alnum:]]+$ ]]; do

edited Nov 17, 2015 at 23:04

answered Nov 17, 2015 at 22:32

chepner

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1 Comment

mikilinux Over a year ago

sorry, but don't work. Try yourself using words "test" and "_test". With test get out of while statement, with _test get out of while statement ...but only with test should be out while statement.

hek2mgl · Accepted Answer · 2015-11-18 10:55:01Z

2

You can use a while true loop and break if the condition is met:

while true; do
    # Read variable
    var=$(dialog ...)

    # Check regex and break if it matches
    [[ "$var" =~ $regexp ]] && break
done

Btw, the regex should be:

regexp="^[[:alnum:]]+$"

edited Nov 18, 2015 at 10:55

answered Nov 17, 2015 at 22:21

hek2mgl

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6 Comments

mikilinux Over a year ago

sorry, but if I use this solution I lose exit code of dialog tool.

mikilinux Over a year ago

Sorry, but don't work: endless loop. Try yourself using words test and _test

hek2mgl Over a year ago

I did. test is valid meaning it would break the lop. _test is not. Are you using the regex I've posted?

chepner Over a year ago

You can save the exit status of dialog with status=$? immediately following the var=... line.

mikilinux Over a year ago

Sorry, endless loop. But it's run properly removing double quote in "Check regex and break if it matches", e.g. with this LOC: [[ $var =~ $regexp ]] && break. Why? Any Idea?

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